1. Field of the Invention
The present invention relates to a polyamideimide-based (PAI-based) film coating composition, in particular a polyamideimide-based film coating composition free from any restricted substance or a substance which might possibly be restricted.
2. Description of Related Art
The polyamideimide resin (PAI) has excellent heat resistance, chemical resistance, mechanical properties at high temperature, wear resistance and sliding characteristics, and therefore is widely used as a binder for various coating compositions such as heat-resistant coating composition, chemical-resistant coating composition, lubricant composition, and heat-resistant and insulating coating composition.
As the polyamideimide resin has a high polarity, only a limited number of solvents are known to have a high affinity for it. Conventionally, as a solvent having a high polyamideimide resin-dissolving property, N-methyl pyrrolidone (NMP) has been mainly used (see, for example, Patent Document 1).
Further, a method of obtaining a solution of polyamideimide resin by synthesizing a polyamideimide resin in a mixture solvent of γ-butyrolactone (GBL) and N-methyl pyrrolidone is known (see, for example, Patent Document 2).    Patent Document 1: JP-A No. 07-97517    Patent Document 2: JP-A No. 2008-133410
However, N-methyl pyrrolidone is recognized as a restricted substance which has a potentially adverse effect on a living body, and therefore its use may be restricted. For such reasons, studies have been made on a replacement substance and examples of the candidate include N-ethyl pyrrolidone (NEP). However, as the structure of N-ethyl pyrrolidone is similar to that of N-methyl pyrrolidone, there may have a risk of similar adverse effect on a living body, and therefore there is also a possibility that its use is restricted depending on further studies in future.
Although γ-butyrolactone can dissolve the polyamideimide resin, since it has a high boiling point of 204° C., the following problem arises. That is, a poor drying property is yielded and a leveling property is deteriorated when only γ-butyrolactone is used as a solvent which impairs coating property, when only γ-butyrolactone is used as a solvent.